The Germany international, who is due to be out of contract at the Veltins-Arena at the end of the campaign, has been linked with an array of European clubs in recent months.
Schalke are unwilling to go down without a fight, though, as they will reportedly make the 22-year-old their highest-paid player.
"We go to the pain threshold," Heidel is quoted as saying by the Liverpool Echo when asked about the ongoing speculation. "We have done everything to show Leon that he is extremely important to Schalke.
"For us, there were always two options: either his contract is extended or he remains here until 2018. It was never an issue for us. We believe that it is best for him to stay here. The ball is now with Leon."
Reports in the German press suggest that a clause will be inserted into any new deal signed by Goretzka that will allow him to join a Champions League club in the future.